When nominally circularly symmetric optical fibre is employed as a long distance transmission path from an optical transmitter to an optical receiver, the departures from perfect circular symmetry of that fibre can be of a sufficient magnitude for the fibre to function as a concatenation of birefringent elements of random relative orientation. Moreover that orientation is liable to change with time.
When polarised light of any particular wavelength is transmitted through a single element exhibiting uniform birefringence, that light is, in general, resolved into two components (modes) propagating with two specific different velocities, and so possessing different transit times of propagation through that element. For each of two particular orthogonal states of polarisation (SOPs), known as the principal SOPs, the light is not resolved into different components, but propagates at a single velocity with a single transit time, i.e. propagates as a single (polarisation) mode. These principal SOPs are aligned with the principal axes of birefringence of the element. For light launched into the element with either one of these two principal SOPs, the SOP of the light remains unchanged in its passage through the element. For light launched into the element with any other SOP, that light is resolved in its passage through the element into two orthogonal components aligned with the principal axes of the element and propagating with different velocities. As the result in the velocity difference, the relative phase of the two components at the far end of the element is generally not the same as that at the launch (input) end, and so the light emerging at the far end generally emerges with an SOP that is different from that with which it entered the element. This characteristic can be conceptualised as the SOP of the light evolving in a cyclic manner in its passage through the element.
When polarised light is transmitted through a concatenation of elements, each exhibiting uniform birefringence, but whose principal axes are not all co-aligned, then, even if that light is not resolved into two components by the first element of the concatenation, it will be so resolved by a later element. Then each of those two elements will itself be resolved into two further components by an element further along the concatenation, and so on. It can be demonstrated that for any such concatenation there exists a specific pair of orthogonal SOPs having the property that light launched with either SOP into the concatenation propagates through it with a single transit time. The transit is faster for one of the SOPs than for the other, and the difference in transit time, the differential group delay (DGD), is a measure of the first order polarisation mode dispersion (PMD) of the concatenation. (The term first order PMD is employed in this specification to denote the DGD in respect of a particular wavelength, thereby excluding from its ambit consideration of second order PMD effects which describe the wavelength dependence of that DGD.) For neither one of this specific pair of orthogonal SOPs is the launch SOP maintained in the passage of the light through the concatenation, and the light emerges at the far end with an SOP that is in general different from that with which it was launched. The emergent SOP for one of the single transit time launch SOPs is orthogonal to the emergent SOP for the other single transit time launch SOP. For any launch SOP that is not one of the single transit time launch SOPs, the emergent light is composed of two components (polarisation modes), generally of unequal amplitude, which have propagated through the concatenation with different transit times, respectively the previously mentioned fast and slow single transit times of the concatenation.
By analogy with the single uniform birefringence element situation, the two single transit time launch SOPs for the concatenation are often referred to as the principal SOPs of the concatenation. Having regard to the fact that for such a concatenation the single transit time input (launch) SOPs are, in general, different from the corresponding output (emergence) SOPs, reference in this specification will be made to input principal SOPs (IPSPs) and to output principal SOPs (OPSPs). From consideration of principles of reciprocity, it will be evident that the IPSPs for one direction of propagation through the concatenation are the OPSPs for the other, and vice versa.
The presence of first order polarisation mode dispersion (PMD) in a transmission pathxe2x80x94the difference between the fast and slow single transit times (DGD)xe2x80x94is liable to be a problem when its magnitude becomes significant compared with the bit period of traffic propagating in the transmission path. Under these circumstances there will be significant pulse broadening at the receiver when bits are launched into the transmission path with an SOP that the transmission path divides into fast and slow single transit time components (modes) of equal power. In principle, this pulse broadening effect could be avoided by taking steps to ensure that the bits are always launched into the transmission path with SOPs matched with one of the IPSPs of the transmission path so that they always propagate, either exclusively with the fast transit time, or exclusively with the slow one, i.e. so that they always propagate in a single mode. However there are difficulties with achieving this in practice. The primary reason for this is that the IPSPs vary with time, and so an active SOP alignment system would be required. Additionally, identification of the IPSPs typically requires access to both ends of the transmission path, and so the active SOP alignment system situated at the transmitter end of the transmission path would require a feedback control signal from the receiver end of that transmission path.
An alternative approach to the avoidance of the problems presented by first order PMD is a compensation approach that involves allowing the bits to be launched into the transmission path with an SOP that the transmission path divides into two components (modes) propagating with different (fast and slow) transit times, and providing an active system at the receiver end which separates the two components, subjects the separated components to controlled variable differential delay to restore synchronisation of the components, and then recombines them.
An example of the PMD compensation approach is described in U.S. Pat. No. 5,659,412. At the receiver, the signal received from the transmitter via the transmission path is fed to a polarisation beam splitter via a polarisation state controller The outputs of the polarisation beam splitter are fed to separate detectors provided with associated clock extraction circuits, and the phase relationship between the two extracted clock signals is determined. The resulting phase difference signal is used to control the polarisation state controller in such a way as to maximise the phase difference. This phase difference is at a maximum when the polarisation state controller is operative to map the OPSPs of the transmission path on to the principal polarisation states of the polarisation beam splitter, and under these conditions the polarisation beam splitter is operative to separate the component of the signal launched into the transmission path that propagates through it with the xe2x80x98fastxe2x80x99 transit time from the component that propagates through it with the xe2x80x98slowxe2x80x99 transit time. In one of the embodiments specifically described, the electrical output of the detector providing the phase-leading clock signal is delayed by the amount corresponding to the measured phase difference between the two extracted clock signals, the DGD, and then the two electrical signals are combined. In the other embodiment specifically described, the two detectors receive only a tapped fraction of the total optical power outputs from the polarisation beam splitter, while the remainder of that power, after the imposition of an optical delay upon the leading component, is optically combined and detected using a third detector. Thus it is seen that the approach of U.S. Pat. No. 5,659,412 necessarily requires the use of at least two detectors capable of operating at the bit rate, some embodiments requiring three such detectors. Moreover operation of the device is complicated by the need to allow for the occurrence of occasions in which either one of the IPSPs of transmission path approaches and passes through coincidence with the SOP of the signal being launched into that transmission path. Under these conditions there is a large disparity in power level between the two outputs of the polarisation beam splitter.
Another option similarly involves allowing the bits to be launched into the transmission path with an SOP that the transmission path divides into two components (modes) propagating with different (fast and slow) transit times, and providing an active polarisation controller at the receiver end. However in this instance the output of the polarisation controller is fed to a birefringent element of fixed, rather than variable DGD. Under these circumstances the adverse effects of first order PMD are not eliminated, but are merely alleviated. This is because whenever the DGD of the transmission path differs from that of the birefringence element, that birefringence element can provide only partial compensation, rather than complete compensation.
An example of this type of partial compensation option is described in the specification of U.S. Pat. No. 5,473,457. This specification describes using a length of polarisation maintaining fibre as the fixed DGD birefringent element, and the data is impressed as amplitude modulation of an optical carrier which is itself frequency modulated in order to provide a control signal at the receiver which can be used for regulating the polarisation controller. This frequency modulation is a significant drawback not least because it adds to the bandwidth of the data.
Another example of this type of partial compensation option is described by T Takahashi et al in an article entitled xe2x80x98Automatic compensation technique for timewise fluctuating polarisation mode dispersion in-line amplifier systemsxe2x80x99, Electronics Letters, Vol. 30, No 4 pp 348-9, Feb. 17, 1994. These authors similarly employ a length of polarisation maintaining fibre as their fixed DGD birefringent element, but generate their control signal for regulating the operation of the polarisation controller by deriving a measure of the magnitude of the frequency component of the detected signal at the receiver that corresponds to half the bit-rate, specifically the frequency component at 5GHz for a 10Gbit/s data rate. A disadvantage of this approach to generating the control signal required for regulating the polarisation controller is the limited response speed that can be obtained for this form of control signal generation. In this context it may be noted that, while the DGD of overhead and land cables may be expected to move with periods of the order of minutes or hours, the IPSPs can be expected to move with periods of the order of seconds, while, in the case of exposed fibres that are subject to being accidentally knocked, the corresponding period is liable to be sub-second.
An object of the invention is to provide a method of PMD compensation using a polarisation controller in association with a variable DGD compensation element, but without having to have recourse to the use of more than one detector for generating a control signal for regulating the polarisation controller and the DGD compensation unit.
A further object of the invention is to provide a method of PMD compensation using a polarisation controller in association with a fixed DGD compensation element, the method affording the capability of relatively fast control response times.
These objectives are accomplished by arranging to modulate the polarisation state of light launched into the transmission path. This modulation is of a form that, when represented on a Poincarxc3xa9 sphere, has an oscillatory rotational component at a frequency f1 about a first axis of the sphere, and an oscillatory rotational component at a frequency f2 about a second axis of the sphere that is orthogonal to the first axis, and where f1xe2x89xa0f2, f1xe2x89xa02f2, and f2xe2x89xa02f1.
Other features and advantages of the invention will be readily apparent from the following description of preferred embodiments of the invention from the drawings and from the claims.